Abstract
Two chemically distinct populations of megacrysts/inclusions are present in hypabyssal-facies kimberlite from Fayette County, S.W. Pennsylvania: A Cr-rich suite and a Cr-poor suite. The Cr-rich suite consists of olivine (Fo 90–93), garnet (1.64-6.14% Cr2O3), diopside (1.89–2.34% Cr2O3), Cr-spinel, and immiscible sulfide-melt products. The suite possesses a limited range of mg, and major element compositions of minerals are similar to those in sheared garnet-lherzolite xenoliths. Minor element variations are consistent with crystal fractionation over a temperature interval from 1310° to 1055°C in the pressure range 48–39 kbar. The Cr-poor suite is more evolved and comprises olivine (Fo 81–85), garnet (<0.1% Cr2O3), picroilmenite (16–37 mol% MgTiO3; 0.05–2.05% Cr2O3), and diopside (<0.1% Cr2O3). This suite represents a relatively advanced stage of fractionation of a Cr-poor melt within the low velocity zone (LVZ). Mixing of these populations and their host melts has resulted in rim compositions of Fo 88–89 on both groups of olivines and reverse-zoned rims on the ilmenite megacrysts (34–51 mol% MgTiO3; 1.4–3.6% Cr2O3).
The Cr-poor suite crystallized from ponded melts segregated from rising diapirs in the LVZ. Crystal fractionation in these melts led to evolved mineral compositions and ilmenite saturation. The Cr-rich suite represents crystal and immiscible sulfide-melt fractionation in a melt within an uprising diapir; the melt/crystal ratio was low and major element compositions in the melt were buffered by ambient mantle compositions (undepleted garnet lherzolite). Uprise of this melt-bearing diapir disrupted evolved, Cr-poor crystal mushes incorporating megacrysts and resulting in mixing of the host melts. Back-reaction and crystallization of the cognate xenocrysts within the hybrid melt resulted in the zoning patterns observed.